linux-kernel - RE: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE

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Date:   Tue, 27 Sep 2022 08:20:39 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'YingChi Long' <me@...lyc.cn>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE_ALIGN

From: YingChi Long
> Sent: 26 September 2022 14:18
> 
> Seems GCC __alignof__ is not evaluated to the minimum alignment of some
> TYPE, and depends on fields of the struct.
> 
>  > Notably I think 'long long' has 4 byte alignment on i386 and some other
>  > 32bit archs.
> 
> C11 _Alignof matches in the case (see godbolt link below). How about
> switch to _Alignof?

__alignof__() returns the preferred alignment, not the minimal one.
So one i386 it returns 8 for 'long long' rather than 4.

Using alignof(struct{long long x;}) does give the required 4.
(as does _Alignof()).

See https://godbolt.org/z/13xTYYd11

	David

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